1. Field of the Invention
This invention relates to an improvement in the process for producing an aluminum-bearing grain-oriented silicon steel strip.
2. Description of the Prior Art
When a grain-oriented electrical steel strip is manufactured from a continuously cast steel slab by a process involving hot rolling followed by a single cold rolling operation, it has conventionally been necessary to use as the starting material a slab containing about 0.04% C and about 3% Si, namely a component system wherein .alpha. to .gamma. phase transformation occurs at a high temperature, and further containing an inhibitor such as Mn, S, Al, N etc. The slab is heated to a temperature of 1,300.degree. C. or higher so as to dissolve the inhibitor and is then hot rolled, whereafter the hot rolled sheet is annealed, cold rolled, decarburization annealed, coated with an annealing separator, recrystallized, and subjected to batch annealing for the purpose of desulfurization and denitrification. This method is characterized by the fact that it employs as the starting material a slab having a component system wherein .alpha. to .gamma. phase transformation occurs at a high temperature, it being possible at the time of secondary recrystallization to obtain the .alpha. single phase system which is essential for secondary recrystallization by decarburization annealing. Another characteristic of the method is that the .gamma. to .alpha. transformation plays an important role in the formation of a finely divided and distributed inhibitor phase, namely AlN, and in the formation of a finegrained matrix.
Although the conventional method provides an electrical steel strip with excellent magnetic properties in the rolling direction, it nevertheless suffers from the following defects.
(1) Since it uses as the raw material a slab having an .alpha. to .gamma. transformation temperature lower than the secondary recrystallization temperature, conversion to .alpha. single phase is necessary prior to batch annealing.
(2) In an Fe--3% Si--0.04% C system, if the amount of Si is increased there will result a decrease in the amount of .gamma. phase. If the amount of C is then increased to obtain the required amount of .gamma. phase, the increases in the Si and C contents combine to cause a degradation in the cold rolling properties. Thus it is difficult to realize a reduction in iron loss by increasing the Si content.
(3) As the slab used as the starting material contains C, N and S, all of which tend to degrade the electrical properties of the steel, it is necessary to carry out purification annealing.
(4) In order to obtain a finely divided and distributed inhibitor phase it is necessary to carry out high temperature heating of the slab and high temperature annealing of the hot rolled sheet.
The inventors carried out experiments in order to find ways to overcome these defects of the prior art as completely as possible. First they prepared slabs by adding Si and Al to pure iron while holding the content of other elements to the lowest level possible, and then subjected the so-prepared slabs to processing under various conditions. Of 500 test pieces used in this experiment only one exhibited secondary recrystallization after a single cold rolling. The experiment was further continued using only a single cold rolling step. As a result it was found that five test pieces, one of which used as its starting material a hot rolled strip containing 0.002% C, 2.65% Si, 0.01% Mn, 0.008% S, 0.020% Al and 0.008% N, had coarse grains with diameters of 40-80 mm. From the shape of the coarse grains it was judged that their formation even in a hot rolled strip having low Mn and S contents was the result of a heat flow in the direction of the sheet surface resembling that in the method of producing single crystals or in the zone melting method.
In succeeding experiments it was found that coarse grains could be produced in the (011) [100] direction using as the starting material a hot rolled strip comprising 0.002% C, 3.00% Si, 0.03% Mn, 0.004% S and 0.02% Al, namely a hot rolled strip with a pure composition. Thus the next subject to be studied in order to realize an industrially practical method was that of producing coarse grains in the (011) [100] direction of converter steel containing impurities. For this purpose, a continuously cast slab of converter steel was remelted and then hot rolled to obtain a hot rolled strip. When this hot rolled strip was subjected to a single cold rolling, there was obtained a hot rolled sheet containing 0.05% Al and having coarse grains in the (011) [100] direction.
However, even in the test pieces which exhibited coarse grains in the (011) [100] direction, when these pieces were annealed in stacked condition, it was found that fine grains occurred at the gaps between the strips, namely at the portions constituting paths for the annealing gas and at the edges of the sheets. Thus it was necessary to make an investigation to determine the conditions under which coarse grains could be obtained in the (011) [100] direction at the portions in contact with the annealing gas, namely at the portions where there was no flow of heat in the direction of the strip surface.
In the case of annealing a stack of as-cold-rolled strips the curl of the strips causes various problems such as making it difficult to stack the sheets in intimate contact with each other, degrading the coating property of the annealing separator, causing the coarse grains in the (011) [100] directions to become too large. In order to alleviate these problems, the cold rolled sheets were subjected to primary recrystallization annealing involving rapid heating and were thereafter coated with the annealing separator and stacked. The primary recrystallization annealing increased the plasticity of the sheets and made them easier to stack. This knowledge has been disclosed in Japanese Patent Public Disclosures Nos. 55-154525 and 56-13433.
In this case, coarse grains were formed in the batch annealing carried out after the primary recrystallization annealing. Therefore, the coarse grains are referred to as secondary recrystallization grains.